The present invention relates to a camera filter which absorbs a portion of the ultraviolet region of the spectrum, while blocking a portion of the red region of the spectrum.
An ultraviolet ray absorbing filter has been conventionally used in order to prevent blue fogging of color film which could otherwise be caused due to the presence of ultraviolet rays. In addition, there have been separately known contrasting red light transmitting filters and color compensating cyanic filters, which act upon the red portion of the spectrum.
The conventional red spectrum transmitting filter is formed of a colored glass or a dyed plastic or gelatin. This filter absorbs light in a range of wavelengths short in comparison with red light, and transmits light at wavelengths which are long in comparison with red light. The spectral transmission curves of such filters are typically within the range between the curves shown in FIG. 3. As is evident from this Figure, the transmission factor of such filters increases dramatically as the wavelength of the incoming light exceeds 600 nm. Filters having characteristics such as shown in FIG. 3 have been primary used in order to bring about a contrasting effect with black and white film.
The conventional cyanic filter transmits light in a range of wavelengths which are short in comparison with red light, and varies the degree of transmission in the red light region. FIG. 4 shows the range of transmission factor characteristics of typical cyanic filters. Such filters are primarily used in order to vary the color balance of a picture photographed with color film, or in order to compensate for an insufficiency in the spectral characteristics of the light source. The filter media is typically a dyed gelatin. Although these filters attenuate the red spectra to some degree, there has not previously been known a filter which blocks red light at wavelengths in excess of those that can be readily perceived by the human eye, resulting in the drawbacks which will be described in more detail below.
Presently, a number of organic dyes very frequently used in various kinds of fabrics strongly reflect red or infrared light. For the most part, this light can not be sensed by human eyes. The spectral composition of a color stimulus in color theory, as seen by human eyes, is as shown in FIG. 5, when indicated using the three spectral stimulus values of the CIE (Committee of International Illumination). The three curves shown in FIG. 5 illustrate the three primary colors x (blue), y (green) and z (red). For example, x is representative of the degree to which human eyes will detect light at a certain wavelength as a blue stimulus. Similarly, z in FIG. 5 is representative of the degree of red sensitivity. With respect to red light having a wavelength of greater than about 650 nm, the degree to which human eyes perceive this light is very limited. Above about 680 nm, light is no longer perceived by the human eyes.
FIG. 6 illustrates the spectral sensitivity curves of the three emulsion layers of color film. As is evident from FIG. 6, the red sensitivity curve exhibits a peak in the neighborhood of 650 nm, indicating that the color film is quite sensitive to incoming light at around this wavelength. Therefore, color film is quite sensitive to red and infrared light which is scarcely perceived by the human eyes. Accordingly, color film strongly senses the reflected red or infrared light referred to above, that is, that light reflected by fabrics impregnated with certain organic dyes.
This influence is outstanding in fabrics perceived as having a green or dark green color. Referring to FIG. 7, the spectral reflection characteristic curves of typical such dyed fabrics are illustrated. As can be determined from FIG. 7, these dyed fabrics very strongly reflect light in the red and near infrared range. When fabrics having reflection factors as indicated in FIG. 7 are photographed, their colors are photographically reproduced either as grey or as a warm color (inclined toward red or orange). That is, although the fabric may appear green or dark green to human eyes, this color is reproduced inaccurately as grey or as a warm color, due to the presence of reflected red and infrared light, to which the color film is particularly sensitive.
In order to prevent this phenomenon, there has presently been adopted a modification method which uses a masking technique during retouching or printing. However, this process is difficult, requires substantial time, and the cost is high. As will be clearly understood from the foregoing drawings, if cloth or fabric which strongly reflects light at frequencies which are not readily sensed by the human eyes is photographed with color film, the color film senses this light more strongly than human eyes, so that cloth which is sensed as being green by the eye is photographically reproduced as a grey or warm color or fog.